DVB-T (Terrestrial Digital Video Broadcasting) is the digital terrestrial television standard adopted by Europe and many other countries. A very important operation in a mobile DVB-T digital terrestrial television demodulator is the estimation of the time-varying channel. If this can be done accurately, then other functions like equalization and inter-carrier-interference cancellation are made simpler.
The conventional method for achieving high Doppler performance for mobile DVB-T is time filtering. In this method, a filter/interpolator is applied in the time dimension on scattered pilots. In a conventional DVB-T implementation, the scattered pilots repeat every four orthogonal frequency division multiplexing (OFDM) symbols. Nyquist sampling theory imposes a theoretical limit on the maximum achievable Doppler performance using time filtering. For example, if the OFDM symbol period is Tu and the guard interval is Tg, then the scattered pilot spacing in time is 4(Tu+Tg). Therefore, the Doppler limit is 0.125/(Tg+Tu) Hertz (Hz). For 8 megaHertz (MHz) 8K OFDM with a guard ratio of ¼, the value of Tu is 896 microseconds (μs), which results in a Doppler limit of 111.5 Hz (for the corresponding 6 MHz version, the limit is 83.5 Hz). In practice, it is very difficult to get close to this theoretical limit because of the sharpness required of the filter. To achieve a sharp filter many filter taps are required. The memory cost of a single tap in the filter is four OFDM symbols and each 8K symbol contains 6817 complex samples. Thus, the hardware cost of such an approach is high.
The Doppler frequency at velocity V is equal to Fc*(V/C), where Fc is the carrier frequency and C is the speed of light. A vehicle traveling at 80 miles per hour (mph) will generate a Doppler frequency of 117 Hz at the top end of the UHF band. Hence, the limits mentioned in the preceding paragraph will be exceeded by fast moving cars and certainly by trains. In addition, the Doppler frequency will be much higher for L band applications. The above theoretical limits can be exceeded if the delay spread of the echo profile (i.e., the length of the channel impulse response) is small. Then, the channel can be estimated using the scattered pilots in each symbol without the need for time filtering. However, the scattered pilots in each OFDM symbol are spaced every 12/Tu Hz. Therefore, the length of the impulse response will be limited to Tu/12 when deriving the channel response from each OFDM symbol. This is inconsistent with the fact that many DVB-T Single Frequency Networks (SFNs) have been designed with guard ratios of Tu/8 and Tu/4 to allow for longer delay spreads. There is a need for methods and structures that are capable of achieving high Doppler performance when estimating a time-varying channel in a DVB-T system without such delay spread limitations.